We propose to continue studies of the ultrastructure of the two distinct auditory end-organs in Old World monkeys, the inner and the outer hair cell systems. Further elucidation of their differences in connectivity, and of their separate roles, will be attempted. The functional significance of the double hair cell system of the Organ of Corti in mammals is a key problem of auditory physiology, but as yet the complex morphology is not completely understood in relation to function. We propose to continue experimental denervation of the efferent systems to the Organ of Corti, so as to determine as fully as possible the origin and route of the apparently separate efferent pathways from the brainstem to the outer and inner hair cells, respectively.